What is Parathyroid Hormone (PTH)?
Parathyroid hormone (PTH) is a crucial regulator of calcium and phosphate metabolism in the human body. It is secreted by the
parathyroid glands, which are typically four in number and situated on the posterior surface of the thyroid gland.
Histological Structure of Parathyroid Glands
The parathyroid glands are composed of chief cells (principal cells) and
oxyphil cells. Chief cells are responsible for the synthesis and secretion of PTH. These cells are small, polygonal, and have a central nucleus with a pale, eosinophilic cytoplasm. Oxyphil cells are larger, less numerous, and have an acidophilic cytoplasm. The glands are encapsulated by a fibrous connective tissue capsule which also sends trabeculae into the gland, providing structural support.
Mechanism of PTH Release
PTH release is primarily regulated by the serum calcium levels through a negative feedback loop. When serum calcium levels drop, the chief cells in the parathyroid glands are stimulated to secrete PTH. Conversely, when calcium levels are high, PTH secretion is inhibited. The secretion of PTH is also influenced by magnesium levels and, to a lesser extent, phosphate levels. Function of PTH
PTH plays a vital role in maintaining
calcium homeostasis. It acts on three main target organs: bones, kidneys, and the intestines. In bones, PTH stimulates osteoclast activity, leading to increased bone resorption and the release of calcium and phosphate into the bloodstream. In the kidneys, PTH promotes the reabsorption of calcium in the distal tubules and inhibits phosphate reabsorption in the proximal tubules. Additionally, PTH stimulates the production of calcitriol (active vitamin D) in the kidneys, which in turn increases calcium absorption from the intestines.
Histological Changes in Hyperparathyroidism
In
hyperparathyroidism, the parathyroid glands undergo hyperplasia or adenomatous changes. Histologically, this can be seen as an increase in the number and size of chief cells. The glands may show nodular formations and an increased number of oxyphil cells. Excessive PTH secretion leads to increased bone resorption, resulting in conditions such as osteitis fibrosa cystica, characterized by fibrous tissue replacement and cystic bone lesions.
Histological Changes in Hypoparathyroidism
In
hypoparathyroidism, the parathyroid glands may appear normal or atrophic. There is a reduction in the number of chief cells, and the glands may show fibrosis. Insufficient PTH secretion leads to hypocalcemia, which can cause muscle cramps, tetany, and convulsions due to increased neuromuscular excitability.
Diagnostic Histological Techniques
Histological examination of the parathyroid glands is usually performed using hematoxylin and eosin (H&E) staining. Immunohistochemistry can be employed to differentiate chief cells from other cell types by staining for PTH. Additionally, electron microscopy can provide detailed images of the secretory granules within the chief cells, further aiding in diagnosis.
Clinical Relevance
Understanding the histology of the parathyroid glands and the role of PTH is crucial for diagnosing and treating disorders related to calcium metabolism. Conditions like primary, secondary, and tertiary hyperparathyroidism, as well as hypoparathyroidism, can significantly impact a patient's quality of life and require precise histological and clinical assessment for effective management.
